The present invention is directed to the modulation of a carrier signal with plural data streams, and more particularly is concerned with quadrature amplitude modulation using digitally encoded data.
One particular application of the present invention relates to the transmission of data over telephone lines by means of modems (modulators/demodulators). Typically, a full duplex modem that offers simultaneous two-way communication at a rate of 1200 bits per second operates in a channel that has about 2 kHz of available useful bandwidth. This bandwidth is insufficient for modulation methods that utilize binary encoding, which encodes one bit of data per signal element, or symbol, and which requires 1 Hz per bit per second for each direction of communication. Accordingly, it becomes necessary to encode more than one bit of information per symbol in order to reduce the apparent data rate and thereby utilize the available channel bandwidth more efficiently. To this end, two bits of digital information are encoded per symbol, so that each symbol represents a unit of information known as a dibit having four possible states. Therefore, an input data rate of 1200 bits per second is transmitted at a symbol rate of 600 baud, i.e., 600 symbols per second, and requires approximately only half the bandwidth of binary signalling.
Various amplitude, frequency and phase modulation techniques have been proposed for dibit signalling. One such technique, and the one to which the present invention is directed, is known as Quadrature Amplitude Modulation. Basically, Quadrature Amplitude Modulation (QAM) is a general term for the simultaneous generation of two suppressed-carrier AM signals whose carriers differ in phase by 90.degree.. A conventional QAM modulator is illustrated in block form in FIG. 1. It comprises two identical filters 10 and 12 that spectrally shape the two baseband data streams {a.sub.n } and {b.sub.n }, respectively. The spectrally shaped signals are used to modulate two periodic, zero-mean quadrature functions of equal amplitude (e.g. sin .omega..sub.c t and cos .omega..sub.c t) in a pair of multipliers 14 and 16. The modulated signals are summed in a summer 18 and the resultant signal is presented to a post-modulation filter 20 that can be used for harmonic suppression and equalization.
In the general case where the two input voltages a.sub.n and b.sub.n are independent and continuous signals with maximum values of .+-.1, the vector that results after their summation could have any magnitude from 0 to 1 and any phase angle. In the special case where the input signals are binary, synchronous non-return-to-zero signals having a value of +1 or -1, the resultant vector assumes one of four possible steady state values of equal amplitude and a phase angle of 45, 135, 225 or 315.degree.. This special case of modulation is called 4-state Quadrature Amplitude Modulation (4-QAM), or, alternatively, Quadrature Phase Shift Keying (QPSK).
In practical applications, the performance of a conventional QAM modulator of the type shown in FIG. 1 is limited. This limitation stems primarily from the fact that a high degree of tracking and symmetry is required of the two analog filters and the suppressed carrier modulators, i.e., they should be ideally matched. This requirement results in the need for complex filters. However, the inability to perfectly match the components in each leg of the modulator, as well as d.c. offset voltages that may be present in the baseband data signals, result in unsuppressed carrier and sideband components. Typically, discrete modulators for baseband modem applications, of the type shown in FIG. 1, have about 34-40 dB carrier suppression and about 40 dB unwanted side-band suppression.
As an alternative to the analog QAM modulator illustrated in FIG. 1, QPSK signals can also be directly generated by digital techniques. One example of such a technique for generating a QPSK signal is disclosed in U.S. Pat. No. 4,049,909. However, techniques such as those shown in the patent typically offer limited flexibility in shaping the resultant frequency spectrum. Accordingly, they may require stringent post-modulation bandpass filtering.